The breeder lied to you about the current weight, and that was wrong. Still, no breeder can guarantee the adult weight. Some yorkies here were tiny when puppies and stayed tiny forever, but other were big puppies, but they didn't grow a lot, so they stayed small as adults. You never know...
I would worry about him being healthy; that the most important thing; The weight, well, I think that's secondary, and although everybody has preferences, you'll find out that is not that important. If you are already falling in love with him, each day you love him more, and you'll love every ounce of him
